We describe a fully conjugated diblock copolymer, poly(3-thiohexylthiophene)-block-(3-hexylthiophene) (P3THT-b-P3HT), which is composed of two equally proportioned blocks that have distinct electronic structure and is suitable for testing in a donor donor acceptor solar cell. Calculations and optoelectronic characterization demonstrate communication between blocks in silico and in solution. In the solid state, powder X-ray diffraction and atomic force microscopy show that the polymer is semicrystalline and has a distinct morphology (relative to a 1:1 physical blend of its constituent homopolymers), both with and without a fullerene-derivative (PC71BM) acceptor. When tested in a device configuration (P3THT-b-P3HT:PC71BM), the deep HOMO level of the P3THT block increases the open circuit voltage (V-oc) to a value that is equal to a P3THT:PC71BM control device. This shows that the V-oc of an equally proportioned ternary organic solar cell can reach the upper limit of the corresponding binary devices. The block copolymer structure is necessary to maintain favorable active layer morphology. The study shows that donor-donor block copolymers are excellent materials for organic solar cells and that a high V., can be obtained even when the HOMO level of one of the blocks is high.